The invention relates to a vibration isolation and cancellation system and method, especially but not exclusively suited for high precision wafer-chip production and inspection equipment.
Products that make use of active vibration isolation and cancellation technology are available commercially, but their degree of effectiveness leaves room for improvement. Active vibration isolation and cancellation technology (also known as air mount technology) for IC production and inspection equipment needs to become more effective with the advancement of the production of chips that require ever-smaller features. Typically, actuators and sensors in an active isolation/cancellation system are not integrated. For instance, U.S. Pat. No. 6,286,644 to Wakui discloses and describes an active vibration isolator wherein in FIG. 7 sensors xe2x80x98P0xe2x80x99 and air spring actuators xe2x80x98ASxe2x80x99 are separate elements.
It is an object of the present invention to provide an active vibration isolation/cancellation system that integrates an actuator and a sensor. The system provides a much improved vibration cancellation behavior. The system is useful for, amongst others, virtual all types of high precision production equipment, e.g., in an air-mount for chip production equipment, high precision microscopes and other high precision equipment. The application of the system, for instance, reduces an important barrier in the quest for chips with smaller features. The invention is based on a notion that an actuator and a sensor in an active vibration isolation/cancellation system can be integrated in such a way that many known performance limits (such as described by gain and phase relations from well known control theory) are removed to a degree where they are no longer a performance limitation.
It is another object of the invention to apply the proposed active vibration isolation/cancellation system in an absolute damper. By combining damper technology with air mount technology, a much improved air mount performance is achieved. The absolute type of (motion) damper of the invention typically but not exclusively comprises two parts that are displaceable relative to one another, at least in the direction to be damped. The first part of this type of damper, that may comprise an actuator coil and a complete sensor assembly, is connected to a first body (typically a mass to be damped). In case the sensor of this type of sensor is of a magnet and coil type, one part, e.g. the sensor-coil, is connected to the first body and the other part, e.g., the sensor-magnet of the invention, may be loosely connected to the first body with respect to motion that needs to be damped. The inventor proposes to attach the sensor-magnet to a reference mass that is supposedly not negatively affected by vibrations from other bodies. The reference mass can be realized by a floating mass (loosely connected to the first body). This floating mass acts in an absolute damper as an absolute reference. The second part of this type of damper, that may comprise an actuator magnet, is stiffly connected to a second body (typically a floor or reference without motion). The second body is only required with respect to the operation of the damper for providing a reaction force as reaction of the action force on the first body by the actuator. It is also possible to use an additional body in which to store the reaction forces. This will give at low frequencies a lower level of performance, but can be preferable in certain cases. In the descriptions above and below, the locations of coils and magnets could be exchanged without affecting the general idea of this invention. Whether or not to swap the locations of the coils and magnets is a discussion that depends on many factors and might change from one implementation to another. Typically however, it is preferred to mount the component with the lowest inertia to the body whose motion is to be damped or controlled
Various aspects of the invention are however also applicable for a relative damper and therefore not limited to the absolute damper. The relative type of (motion) damper typically comprises two parts that are displaceable relative to one another, at least in the direction to be damped. The first part of this type of damper, that may comprise an actuator coil and a sensor coil, is stiffly connected to a first body. The second part of this type of damper, that may comprise an actuator magnet and a sensor magnet, is stiffly connected to a second body.
In a preferred embodiment a Lorenz type coil as a sensor and another one as a actuator is used due to their close-to-ideal performance. By using a specific coil design a potential cross talk between the coils of the sensor and the actuator can be minimized to a level where it can be discarded. A damper provides an opposing force to velocity. Since Lorenz type of coils (also called voice coils) can sense velocities and can provide forces, they are appealing candidates as sensors and actuators. In practical implementations (such as in an active air mount) they are, for that and other reasons, frequently used. Other types of sensors and actuators can also be used. They might however require signal conditioning or other operations to make them applicable. An example of another type of sensor is a laser interferometer.
An important aspect of the invention lies in the observation that the sensor and the actuator of are preferably mounted in such a way that the combination (that is part of the damper) possesses certain relevant properties. One of the relevant properties is that the travel time for a mechanical signal caused by the actuator to the sensor is small. After the actuator induces a mechanical movement the sensor measures the mechanical movement. The sensor and the actuator combination of the invention have an acoustic delay, in a preferred embodiment, of far less than one millisecond (typically faster than 40 microseconds). Also the mass in the direct path between the sensor and actuator should be minimized while the stiffness should be maximized. All these prescriptions can be achieved by placing the sensor and actuator substantially close to each other. Having a limited travel time for the mechanical signal, without any substantial cross talk (between the actuator and the sensor) allows the damper to have a high gain feedback control loop without having any instability. Moreover the damper of the invention preferably, although not exclusively, has an electrical delay (that is between the sensor and the actuator) of less than one microsecond. That means that on detection by the sensor of a signal caused by a mechanical movement, a quick reaction is possible (that is commanding the actuator to generate a force).
It is yet another object of the invention to minimize crosstalk and interferences, in particular between the actuator and the sensor. The inventor found that crosstalk is reduced, amongst others, by using a magnetical type of actuator and a non-magnetical type of sensor. An example of a non-magnetical type of sensor is an optical one. An additional novel manner to achieve a minimum of unwanted cross talk is to place an actuator-coil and a sensor-coil perpendicular relative to each other. By doing so a magnetical field induced by the actuator-coil of the actuator will cause a minimum of induced current in a sensor-coil The sensor coil does not generate a field, so the cross talk concerns only in one way.
By having two instead of one sensor coil, and by using magnets of opposite polarityin the reference body, two sensors are made that give opposite signals when a motion is present, but they give an equal signal when an electrical or magnetical disturbance is present. By subtracting the two sensor signals, the measurement of the motion is amplified, and all common disturbances are cancelled. An equally effective method is to use identical magnet arrangements and an opposite coil winding direction.
The inventor found another way to reduce unwanted cross talk by using a shielding between the actuator and the sensor. When applying a shielding comprising an electrical conductor (e.g., copper shielding), a shielding is achieved for EM waves. The latter type of shielding is also beneficial for reducing negative effects caused by external EM sources (e.g. caused by 50/60 Hz mains supply wiring). A shielding should be applied between the sensor and the actuator and or the external EM source. For instance, the sensor and or actuator can be packed with a copper foil. In order to reduce unwanted magnetical cross talk a magnetical shielding needs to be applied. This can be achieved by, e.g., surrounding the sensor coil(s) with so-called xcexc-metal.
The inventor also found that when covering the damper with a cover against noise the performance of the damper is further increased. That is because the inventor found that acoustical waves act as current inducing disturbances on the coil. The inventor found that it is advantageous to provide an acoustical shield in the absolute type of damper, in particular around the floating reference mass. This shield should have no contact with the reference mass. In a typical but not exclusive embodiment, the sensor-magnet is to be loosely connected to the mass to be damped. When the actuator coil is fed by an ideal current source, a movement of the second body cannot induce a current in the actuator coil. The second body (to which the actuator magnet is mounted) is only present to provide a reaction force of an action force of the actuator (otherwise no movement could be induced to the mass to be damped). When the current source is designed to be close to ideal source, little to no disturbance is expected from a movement of the second body (except for wild movements of the second body whereby collisions occur, in which case there is a construction error).
In a preferred embodiment both the sensor and the actuator are mounted to a body that needs damping but not to each other (e.g., in the prior art both the sensor and the actuator are mounted in an assembly and this assembly is mounted to the object to be damped). The disadvantage of this is that the performance becomes dependent on how well the assembly is mounted to the body to be damped. Even in the best of cases this is always limited. By giving the sensor and the actuator their own interfaces, this dependency is removed. By mounting the sensor and the actuator parallel (instead of serially) with respect to each other, the object to be damped is free of effects caused by deformations in the damper (e.g., by a limited stiffness) since the sensor will only measure movements of the object, without averse affects of deformations in the damper due to actuator forces.
In another preferred embodiment both the sensor""s line of action (or a combination of a multiple sensors) as well as that of the actuator are in the same point and the same direction (this is not the case in the prior art). This embodiment improves the damping of the body since a damping action is performed on the exact location where a disturbance has been measured. The damping characteristics are improved even more when a disturbance can be predicted and an anticipated compensation signal can be fed to a compensation means (e.g., electrical circuitry connected to the actuator) of the damper.
Additional advantages and novel features will be set forth in the description which follows, and in part may become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.